It is common practice for prior art tiltrotor aircraft to have first and second rotors, each disposed in a first and second tilting nacelle. It is further well accepted to mechanically drive the first rotor from a first engine mounted within a first tilting nacelle, and to configure the drive as a reduction gearbox driving the first rotor from the first engine. Alternative engine locations outside the nacelle have been also been proposed in the prior art.
It is also common practice to mechanically connect a first rotor to the second rotor laterally across the wing of the aircraft by means of shafting, called cross-wing drive shafting. The purpose of the cross-wing drive connection is to assure power availability to both rotors in the event of the failure of one engine. This feature helps enable one engine inoperative (OH) helicopter mode flight, and is a fundamental redundancy feature of tiltrotor aircraft. As for any aircraft component, an extremely reliable and light weight system is desired.
A typical flight operation sequence for a tiltrotor aircraft is to take off vertically with nacelles and rotors oriented largely vertically, accelerate to some forward speed using a cyclic pitch command to the rotors mixed with partial tilt of nacelles and rotors. Subsequently, nacelles are tilted further forward until the aircraft adopts an attitude in which the fuselage axis and nacelle axis are generally at the same angle as the flight trajectory. If this action continues progressively and rotation of the rotor axes to the horizontal is accompanied by vehicle acceleration, the lift force required to keep the aircraft in flight will gradually transition from the rotors to the wings, and the rotors will function as propellers when horizontal. As used herein the term “acceleration” includes all changes in momentum with respect to time, including both increases and decreases in velocity.
The rotor of a tilt-rotor aircraft is subject to distinctly different thrust requirements in vertical, helicopter-mode flight and in cruise airplane-mode operation. The same rotor provides both lift and thrust to the airframe in helicopter and cruise modes, respectively. To achieve high efficiency in both regimes, it is desirable to vary the speed of the rotor. The concept and implementation of an optimally variable speed rotor is described in U.S. Pat. No. 6,641,365 to Karem. This and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Aircraft engines, and especially turboshaft engines, provide more power and higher efficiency (in the form of lower fuel consumption) at higher engine rotational speed as opposed to lower engine rotational speed. Engine rotational speed is often measured in rotations per minute, or RPM. Reduction of rotor rotational speed and maintaining engine rotational speed is possible through a transmission gear shift providing two or more gear ratios. The reason for a variable gear ratio is that the range of output shaft speed delivered by current turboshaft engine is not sufficient to meet the speed range required by the rotors. Turboshaft engine output speed is limited by efficiency, engine stall, and other engine operational margins. In such a situation, a gearbox with two or more discrete gear ratios can be highly beneficial. A notable prior art example of changing the gear ratio between the engine and the rotors in a tiltrotor aircraft can be found in the 1950s era Bell™ XV-3 experimental aircraft. In this example, the coordination of engine speed, rotor speed, and the manipulation of the clutch and of the gearshifter was all accomplished manually and with some difficulty by the pilot. Furthermore, a heavy friction clutch was used in the Bell™ XV-3 to allow the operator a wide margin in matching engine and rotor speeds in either gear ratio. This system had poor reliability, a large weight penalty, no automation, and scaled poorly to larger sized aircraft. Consequently, aircraft subsequent to the Bell™ XV-3 have avoided the use of shifting gearboxes.
Thus there is still a need for systems and method that provide high thrust for vertical takeoff and high efficiency for forward flight in rotorcraft, especially tiltrotor aircraft.